Acetic acid incorporation into polyketides

Cytochalasins.—Previous results have shown that the cytochalasins, e.g. cyto-chalasin B (126), are partially polyketide in origin cf. Vol. 7, p. 29 Vol. 6, p. 44). It has now been shown that [2-2H3,2-13C]acetic acid is incorporated into cyto-chalasin B (126) in Phoma exigua and into cytochalasin D in Zygosporium masonii. Labelling of the expected sites by 13C was observed for both metabolites, but, in keeping with results on other acetate metabolites, most of the deuterium was lost, being retained only in the polyketide chain at C-l 1, which is part of the starter acetate unit.102... 

Is a specific Inhibitor of type II fatty acid synthetase In higher plants and . coll 12401. The acetyl-CoA ACP S-acety1-transferase Is the apparent specific site of Inhibition 12411. Another antibiotic, cerulenin (structure not shown) Inhbits -ketococy1-ACP synthetase I In bacteria, fungi, and plants, but also Is Inhibitory to other sites such as polyketide and sterol biosynthesis 1242-2441. Cerulenin and thiolactomycin Inhibited CQ14W-acetate Incorporation Into fatty acids at 150 values of 50 and 4 uM, respectively 12451. Recently cydohexanedlone herbicides have been shown to Inhibit lipid biosynthesis by Inhibition of acetyl-CoA carboxylase 12461. 

The monomeric acridone alkaloids are derived from anthranilic acid and acetate via a polyketide. First studies, in which [ C]-acetate was utilized by cell cultures of Ruta graveolens, indicated that the C-ring of the acridone nucleus was acetate derived. Further research revealed that anthranilic acid is specifically incorporated into the A-ring of rutacridone (Baumert et al, 1982). 

C the acetic acid contained 47% of the activity of the lycopodine, and from the experiment with acetate-2- C the acetic acid contained 21% of the activity of lycopodine. These data are in harmony with the pelletierine but not with the polyketide hypothesis. When it was found (see below) that two molecules of lysine were specifically incorporated into lycopodine (65, 66) the polyketide hypothesis could be dismissed. 

The first biosynthetic studies on azinomycins, which appeared in 2004, reported that sodium acetate labeled at C-1, C-2 or doubly labeled with was each fed to S. sahachiroi at the onset of azinomycin B production. Such experiments revealed that the naphthoate fragment of azinomycin B is of polyketide origin. It was proposed that condensation of one molecule of acetyl-CoA with five of malonyl-CoA leads to linear polyketide 22, catalyzed by a PKS. Further reduction, cyclization, and aromatization of the linear polyketide 22 by the PKS led to the carboxylic acid 23, which would be hydroxylated to give 24, methylated with SAM to give 25 and incorporated into the complete azinomycin B skeleton (Figure 3.10) [19]. 

Polyketides occuring in plants are not always exclusively synthesized from acetate units, but often are of mixed biosynthetic origin. Phenylpropanoid or terpenoid building blocks or sometimes both can be connected with the acetate-derived backbone. In addition, parts of the carbon skeleton can be derived from fatty acids or amino acids. This mixed assembly principle results in a plethora of structurally diverse compounds. Polyketide alkaloids obtained when nitrogen or nitrogen-containing precursors are incorporated into the polyketide backbone will also be discussed. 

From these results it was deduced that any radioactivity in the acetic acid obtained by Kuhn-Roth oxidation of [2-acetate-labeled erythroskyrin must be derived from C(26). The acetic acid contained 4.45% of the total activity, and this was shown by the Schmidt degradation procedure to be located in the methyl group. The acetic acid obtained by Kuhn-Roth oxidation of [1- " C]- and [2- C]malonate-derived erythroskyrin was inactive. These results proved that the biosynthesis of erythroskyrin involved the condensation of valine and a polyketide moiety. More specifically, the results were taken as evidence that erythroskyrin is biosynthesized from valine [N(l) and C(4) to C(8)], one acetate [C(25) and C(26)], and nine malonate [C(2), C(3), C(9) to C(24)] units. This conclusion must, however, be considered as tentative, particularly as no direct evidence was obtained on the incorporation of from the various substrates into C(2) and... 

Feeding experiments with isotopically labeled precursors have shown that many NR fungal polyketides are formed by the use of advanced starter units. In the classic case of norsolorinic acid 7 biosynthesis, it has long been known that hexanoate forms the starter unit. Differential specific incorporation of acetate into the early and late positions in compounds such as citrinin 3 have been used to argue that these compounds may have been formed by more than one PKS so that one PKS makes an advanced starter unit, which is passed to a second PKS for additional extension. 

C16 polyketides respectively) with methionine providing, respectively, two and three Cl units. Results with [l,2- C2]acetate more rigorously defined the constitution of the polyketide fragment of cytochalasin D as that shown in (148). The reported intact incorporation of palmitic acid into brefeldin A (subsequently refuted) prompted the examination of l- C/ C-labelled palmitic and myristic acids together with butyric acid as cytochalasin D precursors. In each case incorporation was through C-1 labelled acetate resulting from fragmentation by way... 

In E. coir. The conversion of pimelate into pimeloyl-CoA does not take place in E. coli. Preliminary labeling experiments with " C02, which ruled out the symmetrical pimelate as intermediate, were confirmed by further studies using C-labeled acetate, with E. coli strains overexpressing the biotin operon. Nuclear magnetic resonance (NMR) analysis of DTB or biotin revealed that carbons 3, 5, 7 and 2,4,6 were obtained from C-1 and C-2 of acetate, respectively (Figure 5). Sanyal etal confirmed using [l,2- C]acetate that intact acetate units were incorporated, consistent with a classical fatty acid or polyketide pathway. The results of the two groups differ, however, on an important point. Sanyal etal did not observe C incorporation at C-1 or C-10 with 1- or... 

The relatively high incorporations of tracer molecules into secondary metabolites in micro-organisms has enabled detailed studies to be made of the biosynthesis of the fungal metabolites patulin (10) and multicolic acid (79). Carbon-14 work has demonstrated the polyketide origin of patulin, and the biosynthesis of multicolic acid from acetate has been studied by C-n.m.r. spectroscopy. The biosynthesis of patulin has been investigated extensively at the enzymology level. In the first... 

In one of the first demonstrations of the use of doubly C-labelled acetate in a biosynthetic problem, Holbcer and co-workers 82) have shown that the biosynthesis of multicolic acid (79) in Penicillium multicolor proceeds via oxidative cleavage between C-4 and C-5 of the poly-ketide derived aromatic acid (191). From 1,2- C-acetate feedings, and the magnitudes of the C- C-couplings in the C-n.m.r. spectra of the metabolite, it was established that the acetate residues C-2—C-5, C-6—C-7, C-8—C-9 and C-4—C-10 were incorporated intact into the acid (Scheme 17). These data are only compatible with a biosynthetic origin by oxidative cleavage of the polyketide derived aromatic precursor (191). 

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